1. Field of the Invention
The present invention relates to the design of Ethernet passive optical networks. More specifically, the present invention relates to a method and apparatus for accommodating TDM traffic in an Ethernet passive optical network with reduced latency.
2. Related Art
In order to keep pace with increasing Internet traffic, network operator shave widely deployed optical fibers and associated optical transmission equipment, substantially increasing the capacity of backbone networks. This capacity increase in backbone networks, however, has not been matched by a corresponding increase in the capacity of access networks. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks still presents a severe bottleneck in delivering high bandwidth to end users.
Among the different developing technologies, Ethernet passive optical networks (EPONs) are one of the best candidates for next-generation access networks. EPONs combine ubiquitous Ethernet technology with inexpensive passive optics, offering the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. With the high bandwidth of optical fibers, EPONs can accommodate broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, because Ethernet frames can directly encapsulate native IP packets with different sizes, whereas ATM passive optical networks (APONs) use fixed-size ATM cells and consequently require packet fragmentation and reassembly.
Typically, EPONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and business or residential subscribers. The “first mile” is generally a logical point-to-multipoint network, where a central office services a number of subscribers. For example, an EPON can adopt a tree topology, wherein one fiber couples the central office to a passive optical splitter/combiner. The passive optical splitter/combiner divides and distributes downstream optical signals to subscribers and combines upstream optical signals from subscribers (see FIG. 1). In the following description, “downstream” refers to the direction from an OLT to an ONU, and “upstream” refers to the direction from an ONU to an OLT.
Transmissions within an EPON are performed between an optical line terminal (OLT) and optical network units (ONUs) (see FIG. 2). The OLT generally resides in the central office and couples the optical access network to a metro backbone, which is an external network belonging, for example, to an Internet Service Provider (ISP) or a local exchange carrier. An ONU can reside either at the curb or at an end-user location, and can provide broadband voice, data, and video services. ONUs are coupled to a one-by-N (1×N) passive optical coupler, where N is the number of ONUs, and the passive optical coupler is coupled to the OLT over a single optical link. One may use a number of cascaded optical splitters/couplers to increase the number of ONUs. This configuration can significantly save the number of fibers and amount of hardware.
Communications within an EPON include downstream traffic (from OLT to ONUs) and upstream traffic (from ONUs to OLT). In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, data frames are broadcast by the OLT to all ONUs and are selectively extracted by their destination ONUs. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler to the OLT.
Because EPONs are asynchronous packet-switched networks, currently there is no EPON implementation which can seamlessly carry time-division multiplexed (TDM) traffic while satisfying stringent quality-of-service (QoS) requirements. TDM is a technique for multiplexing a number of low-speed digital channels onto a high-speed channel by assigning fixed periodical timeslots to each low-speed channel. A TDM channel has a fixed bandwidth and typically has stringent latency requirement. For example, a T1 carrier provides a TDM channel at 1.544 Mbps. Another example is the E1 carrier which provides a TDM channel at 2.048 Mbps. To guarantee certain QoS, TDM channels usually impose jitter and latency limits. Currently, EPON technologies do not provide solutions for meeting these requirements.
Hence, what is needed is a method and an apparatus for accommodating TDM traffic in an EPON with reduced jitter and latency.